1. Field of the Invention
The present invention relates to a color image forming apparatus, and more particularly to a color image forming apparatus that finally transfers and fixes a toner image onto a recording sheet by an electrophotographic method.
2. Description of Related Art
In an electrophotographic color image forming apparatus that performs binary component developments by use of developers, each composed of toner and carriers, conventionally, the charge amount of toner depends on the color of the toner, that is, depends on whether the toner is Y (yellow), M (magenta), C (cyan) or K (black), and also depends on the toner concentration of the developer. Therefore, in order to achieve a target toner adherence amount of the colors on a toner image bearing member, such as a photosensitive drum or an intermediate transfer belt, either of the following methods is conventionally carried out; 1) a method wherein while the toner concentrations of the respective colors are fixed to a specified value, developing bias voltages for developments of the colors are adjusted; and 2) a method wherein while developing bias voltages for developments of the respective colors are fixed to a specified value, the toner concentrations of the colors are adjusted.
In the method 1), since the toner concentrations of the colors are set to a specified value, the toner concentrations never become out of a predetermined range from a lower limit and an upper limit, and trouble such as smudge and carrier consumption can be avoided. However, in the method 1), different developing bias voltages must be applied for developments of different colors. Accordingly, when the color image forming apparatus is of a tandem type that transfers toner images from four juxtaposed photosensitive drums one after another onto an intermediate transfer belt (first transfer) to combine the four color images into a full-color image, first transfer voltages for the first transfers of the respective colors must be set separately from one another because the toner concentrations of the colors are different from each other. Further, when the first transfer voltage to be applied to a photosensitive drum disposed downstream with respect to a moving direction of the intermediate transfer belt must be set higher than the first transfer voltage to be applied to a photosensitive drum disposed upstream, the amount of reversely transferred toner increases due to the higher transfer voltage at the downstream transfer point, and toner consumption and toner waste unnecessarily increases. Moreover, in performing second transfer from the intermediate transfer belt to a recording sheet, the optimal transfer conditions are different from color to color, and it is impossible to set an optimal second transfer voltage for all the four colors.
In the method 2), as described in Japanese Patent Laid-Open Publication No. 2001-147580, while the developing bias voltages for developments of the respective colors are fixed to a specified value, the toner concentrations are adjusted so as to achieve a target toner adherence amount. Therefore, it is possible to set the developing bias voltages for developments of the respective colors to the same value. In the method 2), however, because the toner concentrations of the colors are different, it is difficult to determine an optimal developing bias voltage common to all the colors beforehand. Further, if it is found out that the toner concentration of a color must be set to a value out of the predetermined range to achieve the target toner adherence amount, the target toner adherence amount or the target development potential must be changed. More specifically, if it is found out that the toner concentration must be set less than the lower limit, the target toner adherence amount shall be changed to a higher value. If it is found out that the toner concentration must be set beyond the upper limit, the target development potential shall be raised. When the developing bias voltages for all the colors are set to the same value, naturally, it often occurs that the toner concentration for any of the colors must be set out of the predetermined range.